Several methods are known in the art for sensing thicknesses or weights of materials. These can be based on nuclear radiation, X-ray methods, optical methods based on laser triangulation, acoustic methods based on time-of-flight, etc.
Due to the rising importance of millimeter-wave communication technology in the field of consumer electronics, attention to millimeter-waves and terahertz signals has been shown for sensing applications. Millimeter-wave and terahertz based sensing applications have certain advantages with respect to the industry-standard nuclear based techniques. Radioactive solutions have high indirect cost-of-ownership due to the harmfulness of the used radiation type, whereas millimeter-waves and terahertz radiation is non-ionizing. In addition, current trends in safety regulations (like for instance in Europe) are guiding consumers to be critical while obtaining licenses for new nuclear systems which are based on ionizing radiation.
The corresponding wavelengths are at maximum a few millimeters, yielding interesting applications for precise and accurate sensing. An important condition is that the signal's phase values are sufficiently reliable to use them as the physical parameter on which the sensing concept is based.
U.S. Pat. No. 5,886,534 provides a millimeter wave sensor for non-destructive inspection of thin sheet dielectric materials. To enable the latter US'534 uses a single antenna which generates a signal which is reflected from the surface of the thin sheet dielectric material. This means that for cases where high amounts of reflections on the thin sheet dielectric material occurs (e.g. when the material has a high index of refraction), the sensor mainly measures the surface state of the thin sheet.
DE102011002413 describes a backscatter imaging device comprising a lock-in amplifier that generates an output signal by correlating an intermediate frequency signal with a correlation signal. DE'413 uses a heteronymous receiver or conventional correlation reception systems which generate a received signal based on radiation emitted from a transmitter. For this purpose, the device comprises a transmitter for emitting a radiation based on a carrier signal having a carrier frequency. The device generates a received signal and an intermediate frequency signal by mixing the received signal with a tuning signal, wherein the tuning frequency is different from the carrier frequency.
Hence there remains a need for millimeter wave sensing methods and devices.